Effect of surface steps on chemical ordering in the subsurface of Cu(Au) solid solutions

Atomic steps are typically assumed to only influence surface phenomena of solids. In contrast, here we show, using in situ atomic-resolution electron microscopy observations and atomistic modeling, the pronounced effect from surface steps in inducing compositional and structural evolution in the subsurface of a Cu(Au) solid solution. We find that Au surface segregation results in a stacked sequence of Cu-Au ordered phases in the subsurface. The presence of a monatomic step at the surface induces the formation of an antiphase boundary that extends from the surface step to deeper layers by maintaining the same composition profile associated with each terrace. The bunching of surface steps induces chemical disordering of the Cu-Au ordered phases in the subsurface region of the bunched steps. These results demonstrate the instant propagation of surface dynamics of atomic steps into the subsurface region and can find broader applicability in utilizing surface defects to tune the composition and structure in the subsurface of alloys.

Figure 1

Identification of the Au-segregation profile and compositional ordering in ${\mathrm{Cu}}_{20}{\mathrm{Au}}_{80}$ solution at $T=350{}^{\circ }\mathrm{C}$ and $p{\mathrm{H}}_2=1\times {10}^{-3}\mathrm{Torr}$. (a) HRTEM image of a (100) plane showing the formation of ordered phases in the subsurface. Upper inset: zoomed-in view of the area marked by the red rectangle. Right inset: simulated HRTEM image using the structure model in (c). (b) [010] projected views of ${\mathrm{Cu}}_3\mathrm{Au}\text{−}L{1}_2$, $\mathrm{CuAu}\text{−}L{1}_0$, and $\mathrm{Cu}{\mathrm{Au}}_3\text{−}L{1}_2$. (c) DFT-optimized structure. (d) Au-composition profile based on the stacking sequence of the ordered phases identified from (a)–(c).

Figure 2

In situ atomic-scale imaging of surface-step motion induced composition/structure evolution in the subsurface of Cu(Au) at $T=350{}^{\circ }\mathrm{C}$ and $p{H}_2=1\times {10}^{-3}\mathrm{Torr}$. (a)–(d) Migration of a monatomic step (Supplemental Material in situ TEM movie 1 [24]). The surface consists of two terraces separated by a monatomic step. Insets: enlarged view of the areas marked by dashed red boxes, the inset yellow rectangles mark the same atomic layer. (e)–(h) Migration of two bunched steps (Supplemental Material in situ TEM movie 2 [24]). The surface consists of three terraces separated by two monatomic steps. The yellow rectangles mark the subsurface region of the narrow terrace in the middle. The red arrows mark the two bunched steps. (i)–(l) Migration of three bunched steps (Supplemental Material in situ TEM movie 3 [24]). The surface consists of three terraces separated initially by a double-layer atomic step and one monatomic step, as indicated by the red arrows. The dashed red boxes mark the subsurface region of the bunched steps. The yellow rectangles mark the same atomic layer in the subsurface of the wide terrace.

Figure 3

Au segregation at stepped surfaces of the Cu(Au) at $T=350{}^{\circ }\mathrm{C}$ and $p{\mathrm{H}}_2=1\times {10}^{-3}\mathrm{Torr}$. (a) (100) surface consisting of a monatomic step. Atomic layers marked with the rectangles of the same color have the same phase and composition. The monatomic step results in antiphase boundary formation by the misalignment of the ordered phases in the subsurface of the two terraces. (b) 3D illustration of the atomic structure in (a). (c) (100) surface consisting of two wide terraces and one narrow terrace separated by two monatomic steps. The subsurface of the narrow terrace is denoted by the dashed lines. The dashed red rectangle shows that the deeper layers of the subsurface of all the three terraces exhibit the same ${\mathrm{Cu}}_3\mathrm{Au}\text{−}L{1}_2$ ordering. (d) Three-dimensional (3D) illustration of the atomic structure in (c).

Figure 4

Ex situ HAADF STEM micrographs of the Cu(Au) solid solution annealed at 350 °C in vacuum. (a) HAADF image illustrating the characteristic ${\mathrm{Cu}}_3\mathrm{Au}\text{−}L{1}_2$ ordering in the relatively deep atomic layers. (b) HAADF image showing the presence of two wide terraces separated by a monoatomic surface step (marked by the green line). The red rectangles mark the two atomic layers showing the $L{1}_2$ ordering of Cu and Au atoms in the subsurface region of the two terraces. The misalignment of the two atomic layers by one atom spacing along the monoatomic step direction indicates the tendency to form an antiphase boundary. (c) HAADF image showing the presence of surface ledges in a region far away from the sample edge, where the surface ledges are dominated by atom columns with brighter contrast, indicating the tendency to form pure Au atom columns along the ledges. The insets in (a)–(c) are the corresponding structure models of the regions marked by the red boxes.

Figure 5

(a) Atomic model of the (100) ordered segregation profile. (b) Ten alternative random solid solution models of the Cu-Au alloy. (c) DFT-computed relative enthalpies of ten alternative surface profiles of the random solid solutions in (b) with respect to the ordered segregation profile in (a).

Figure 6

(a) Schematic of the stepped surface with an antiphase boundary (colored in blue, top panel) and a stacking fault (colored in red, lower panel) in the subsurface. (b) Atomic models of the stepped surface with an antiphase boundary (blue parallelogram) beneath the monatomic step and the stepped surface with a subsurface stacking fault (red parallelogram) beneath the surface terrace. (c) DFT-calculated energies of the antiphase boundary and stacking fault as a function of the terrace width. Their intersection corresponds to the critical terrace width, beyond which the antiphase boundary formation is energetically more favored over the stacking fault.